2. Castiel - Supernatural
Once upon a time there were two super damaged brothers who met a delightfully adorkable angel. That angel was named Castiel, and when he showed up, nothing could ever go back to the way things were. You see, before Cas, the show had just been about the Winchester brothers. Sure, minor supporting characters came and went, but they never stayed for very long, and the show was ultimately supported by Sam and Dean (and their brotherly man love). When Cas first appeared in the third season, he was a sporadically appearing character at best. He popped in from time to time to give the Winchesters cryptic messages and save their asses every once in a while. But then the production team realized what a gem they had. Because audiences loved Mischa Collins, and wanted him to be around as much as humanly (or angelically) possible. Over time, Castiel took on a larger role, and the duo became something of a threesome (if you'll forgive the expression). He went from heavenly ally to enemy and back again, and now he's finally human. But regardless of his allegiance and moral standing, Castiel has been a far more central character than any of the other supporting characters that we've seen on Supernatural.